1. Field of the Invention
A measure of the fluidic permeability of a porous material is often necessary for product evaluation. It can often be used to determine other characteristics of the material. For example, in a cotton fiber batt, the bulk density of the material can be related to the material's permeability to fluid flow. The density measurement can then be used in conjunction with other measurements for determining properties such as micronaire and moisture content.
Cotton fiber moisture content is one of the most significant parameters needed for process control and performance optimization of commercial cotton gins. The moisture level affects the fiber length, fiber length uniformity, and the removal of trash from the cotton lint. It is necessary to correlate moisture readings with permeability measurements of both the seed cotton and the cotton lint in order to obtain an accurate measure of moisture content.
2. Description of the Prior Art
Most conventional systems for measuring permeability of a material require totally confining a static volume of the material in a chamber during the measurement, thereby fixing its dimensional shape. A fluid is then passed through the material and the flow and pressure drop across the material are measured and used to determine the permeability. This method is not well suited for making high-speed permeability measurements in an industrial process in that it involves a continuous side-stream sampling of the feed stream. Such a system is shown for example by Rose in U.S. Pat. No. 4,506,542. The sample chamber therein comprises four fluid impervious sides, and front and back porous barriers. The pressure and flow rate of the fluid is regulated and measured on either side of the porous test material.
Various devices for measuring the permeability of the moving porous web have arisen from the cigarette paper manufacturing industry. For example, in U.S. Pat. No. 4,311,037, Gotchel et al. passes a paper web over a sensing head having one or more apertures through which a vacuum is drawn on the web. A differential pressure transducer compares the static pressure within the aperture to a reference pressure. The output of the transducer is used to generate a continuous reading of the pressure drop across the web representative of the permeability. In U.S. Pat. No. 4,471,649, Cranshaw shows holding one side of a moving sheet over the inlet of a first gas-flow chamber by directing a gaseous medium from the outlet of a second flow chamber against the opposite side of the sheet. The permeability of the sheet to the gaseous medium is measured by a pressure transducer connected across a low-impedance, multi-capillary laminar-flow device. Hester et al. in U.S. Pat. No. 4,495,796 discloses a device for monitoring both the pressure drop and the flow rate of a fluid passing through a sensing head in contact with a moving web. The volumetric flow rate is determined by directing the fluid through a laminar-flow element, sensing the pressure differential across the element by means of a first transducer, and generating a signal analogous to the flow rate. A signal representing the pressure drop across the web as measured by a second transducer is used in conjunction with the flow rate signal to develop a signal representing permeability.
It is apparent from the above patents that the art-established method for determining permeability requires measuring the pressure drop across the entire thickness of the material. Moreover, in no case does the prior art dealing with permeability measurement of a moving material address the effect on the measurement of the fluid medium in the pores or interstices of the material. In the case of paper webs such as disclosed in Gotchel et al., Cranshaw, and Hester et al., the effect of the medium in the pathways between the fibers would obviously be minor in comparison to that in a cotton batt. It can be appreciated that in materials of relatively low bulk density, the density of fluid medium could have a substantial influence on the apparent permeability of the material. In such cases, the effects of the temperature, pressure, and humidity on the density of the medium become significant.